"When we consider a given country politico-economically, we begin with its population, its distribution among classes, town, country, the coast, the different branches of production, export and import, annual production and consumption, commodity prices etc.

It seems to be correct to begin with the real and the concrete, with the real precondition, thus to begin, in economics, with e.g. the population, which is the foundation and the subject of the entire social act of production. However, on closer examination this proves false. The population is an abstraction if I leave out, for example, the classes of which it is composed. These classes in turn are an empty phrase if I am not familiar with the elements on which they rest. E.g. wage labour, capital, etc. These latter in turn presuppose exchange, division of labour, prices, etc. For example, capital is nothing without wage labour, without value, money, price etc. Thus, if I were to begin with the population, this would be a chaotic conception [Vorstellung] of the whole, and I would then, by means of further determination, move analytically towards ever more simple concepts [Begriff], from the imagined concrete towards ever thinner abstractions until I had arrived at the simplest determinations. From there the journey would have to be retraced until I had finally arrived at the population again, but this time not as the chaotic conception of a whole, but as a rich totality of many determinations and relations...."

N.B. the words Vorstellung and Begriff have Hegelian connotations that are worth bearing in mind; Hegel's use of Vorstellung is sometimes translated as 'picture thought', as it means an inadequate conceptual representation that remains distinct from its object. Begriff refers to the Hegelian Concept (sometimes translated as Notion) which basically means the metaphysical mechanics underlying reality. Previous philosophy understood things as Vorstellungen, Hegel grasps the Begriff (Spirit - human self-consciousness, society, philosophy etc. - thus effectively becomes the self-consciousness of the universe). Borgeouis economics only got the surface appearances of capitalism, Marx gets the essential categories and concepts underlying it (this leads characters like Lukacs to posit Marxist thought, when actualise in the Party, as a kind of self-consciousness).

...that's an awful explanation; I'm writing this at work. I'll try and clarify next Thursday if required.